Such an arrangement is known from the German Auslegeschrift No. DE-B 1,293,873.
These circuit arrangements are used to detect the loop condition of subscriber's lines.
In some private telephone systems (PABX) so-called earthing buttons are used to enable earth return signalling from the subscriber's set to the telephone exchange during the call condition. By pressing the earthing button of the telephone set, the a-wire of the subscriber's line is connected to earth. This signal can be detected in the telephone exchange by a detection circuit connected to the supply circuit of the b-wire, as described in the German Offenlegungsschrift No. DE-A 2,547,880. This detection method has the drawback that the detection circuit for the earth button-signalling is also responsive to the interruption of the subscriber's loop.
It is an object of the invention to extend a circuit arrangement of the afore-mentioned type in a similar way, thus enabling the detection of the common-mode component of the currents in the two conductors. In practice, this may inter alia be used for detecting the earth button-signalling. The advantage is obtained that no confusion will occur with the interruption of the subscriber's loop, so that this detection of the earthing button-signalling is unambiguous.